header {
	margin: 0 auto;
	width: 800px;	
	border: 1px solid black;
	padding: 2px;
}

.field {
	margin: 0 auto;
	width: 800px;	
	height: 600px;
	background: yellow;
	border: 1px solid black;
	margin-top: 10px;
}

.ball {
	height: 50px;
	width: 50px;
	background: red;
	position: absolute;
}